Mobility of cesium through the Callovo-Oxfordian claystones under partially saturated conditions

Cadmium pollution exacerbated by drought: Insights from the nanoscale interaction at the clay mineral surface

Drought is a global environmental problem, while the effect of drought-induced unsaturation on the fate of heavy metal ions is still poorly understood, particularly the lack of mechanistic information at the molecular level. This study used molecular dynamics simulations to investigate nanoscale interactions at the montmorillonite surface under different moisture conditions. Compared to the saturated condition, drought increased the amounts and strength of Cd2+ ions adsorbed on the montmorillonite (MMT) surface while decreased the diffusivity, which was especially obvious in extreme drought conditions (θv=21%-7%). This is closely related to the compressed electric double layer, overcompensation of surface charge, and increased ion pair interactions, resulting from the confinement of water films under drought stress. Further analysis showed that the decrease of hydration effect was responsible for the exacerbated cadmium pollution. Therefore, this study may break the stereotypes about the interactions between heavy metal ions and soil minerals. The results suggest that water management (e.g., irrigation) may be prioritized before beginning heavy metal remediation.

Resolving experimental biases in the interpretation of diffusion experiments with a user-friendly numerical reactive transport approach

The reactive transport code CrunchClay was used to derive effective diffusion coefficients (De), clay porosities (ε), and adsorption distribution coefficients (KD) from through-diffusion data while considering accurately the influence of unavoidable experimental biases on the estimation of these diffusion parameters. These effects include the presence of filters holding the solid sample in place, the variations in concentration gradients across the diffusion cell due to sampling events, the impact of tubing/dead volumes on the estimation of diffusive fluxes and sample porosity, and the effects of O-ring-filter setups on the delivery of solutions to the clay packing. Doing so, the direct modeling of the measurements of (radio)tracer concentrations in reservoirs is more accurate than that of data converted directly into diffusive fluxes. While the above-mentioned effects have already been described individually in the literature, a consistent modeling approach addressing all these issues at the same time has never been described nor made easily available to the community. A graphical user interface, CrunchEase, was created, which supports the user by automating the creation of input files, the running of simulations, and the extraction and comparison of data and simulation results. While a classical model considering an effective diffusion coefficient, a porosity and a solid/solution distribution coefficient (De-ε-KD) may be implemented in any reactive transport code, the development of CrunchEase makes it easy to apply by experimentalists without a background in reactive transport modeling. CrunchEase makes it also possible to transition more easily from a De-ε-KD modeling approach to a state-of-the-art process-based understanding modeling approach using the full capabilities of CrunchClay, which include surface complexation modeling and a multi-porosity description of the clay packing with charged diffuse layers.

Capillary suction under unsaturated condition drives strong specific affinity of ions at the surface of clay mineral

Mineral-solution interface is of great importance in many soil and geochemical processes as well as industrial applications. Most relevant studies were based on saturated condition and given the corresponding theory, model, and mechanism. However, soils are usually in the non-saturation with different capillary suction. Our study presents substantially different scenery for ions interacting with mineral surface under unsaturated condition using molecular dynamics method. Under partially hydrated state, both cations (Ca²⁺) and anions (Cl^-) can be adsorbed as outer-sphere complexes at the montmorillonite surface, and the number significantly increased with the increase of unsaturated degree. Ions preferred to interact with clay mineral instead of water molecules under unsaturated state, and the mobility of both cations and anions substantially decreased with the increase of capillary suction as reflected by the diffusion coefficient analysis. Potential of mean force calculations further clearly revealed that the adsorption strength of both Ca²⁺ and Cl^- increased with capillary suction. Such an increase was more obvious for Cl^- compared to Ca²⁺, despite the adsorption strength of Cl^- was much weaker than Ca²⁺ at a certain capillary suction. Therefore, it is the capillary suction under unsaturated condition that drives the strong specific affinity of ions at the surface of clay mineral, which was tightly related to the steric effect of confined water film, the destruction of EDL structure, and the cation-anion pair interaction. This suggests that our common understanding of mineral-solution interaction should be largely improved.

Influence of non-equilibrium and nonlinear sorption of 137Cs in soils. Study with stirred flow-through reactor experiments and quantification with a nonlinear equilibrium-kinetic model

This paper addresses the modelling of cesium sorption in non-equilibrium and nonlinear conditions with a two-site model. Compared to the classical K_d approach, the proposed model better reproduced the breakthrough curves observed during continuous-flow stirred tank reactor experiments conducted on two contrasted soils. Fitted parameters suggested contrasted conditions of cesium sorption between 1) equilibrium sites, with low affinity and high sorption capacity comparable to CEC and 2) non-equilibrium sites, with a fast sorption rate (half-time of 0.2-0.3 h), a slow desorption rate (half-time of 3-9 days) and a very low sorption capacity (0.02-0.04% of CEC). Comparison of EK sites densities with sorption capacities derived from the literature suggests that the EK equilibrium and kinetic sites might correspond to ion exchange and surface complexation of soil clay minerals respectively. This work stresses the limits of the K_d model to predict ¹³⁷Cs sorption in reactive transport conditions and supports an alternative non-equilibrium nonlinear approach.

Water and ion diffusion in partially-water saturated compacted kaolinite: Role played by vapor-phase diffusion in water mobility

Diffusion is the main transport process of water and solutes in clay-rich porous media owing to their very low permeability, so they are widely used as barriers against contaminant spreading. However, the prediction of contaminant mobility can be very complicated when these media are partially water-saturated. We conducted diffusion experiments for water (HTO and HDO) and ions (²²Na⁺ and ¹²⁵I^-) through partially water saturated compacted kaolinite, a weakly charged clay material, to quantify the distinct diffusive behavior of these species. The osmosis method was used to set kaolinite samples at 67, 86 and 100% saturation. The results showed that desaturation led to a sharp decrease in diffusive rates by factors of 6.5, 18 and 35 for HTO, ¹²⁵I^- and ²²Na⁺, respectively, from 100 to 67% of the degree of saturation. Thus, to interpret water diffusivities, we proposed a model taking into account the diffusion of water in both gas and liquid phases, using diffusion data obtained for ions, considered as inert species. This model was capable of properly predicting water diffusive flux, especially at a low degree of saturation (67% saturation), for which the assumption made for the occurrence of air phase continuity throughout the sample appears to be more relevant than at 86% saturation.

Cs selectivity and adsorption reversibility on Ca-illite and Ca-vermiculite

For understanding and predicting the Cs behavior in soils and groundwater, Cs adsorption properties of illite and vermiculite were investigated under various pH conditions and Cs concentrations. Cs adsorption and desorption experiments have been conducted with Ca-homoionic illite and Ca-vermiculite in CaCl₂ solution with an ionic strength of 0.03 and of 0.06 mol.L^-1 respectively, by focusing on cation exchanges between Cs, proton and calcium at thermodynamic equilibrium. Ca-illite displayed more affinity for Cs than Ca-vermiculite. Cs adsorption was non-linear for both clay minerals and a multi-site exchange model approach was adopted to interpret and model adsorption isotherms. Each mineral reactivity was described by their sorption site properties expressed by their exchange capacity and ionic selectivity. High-selective and low-capacity sites were shown to control Cs uptake at concentrations below 10^-8-10^-7 mol.L^-1 for both Ca-illite and Ca-vermiculite. Three high-capacity sites dominated Cs adsorption at higher concentrations. Cs adsorption reversibility was demonstrated for illite at Ca concentrations ranging from 5 10^-2 to 10^-9 mol.L^-1. The partial irreversibility of Cs adsorption onto vermiculite at Cs concentrations greater than 10^-5 mol.L^-1 was related to interlayer collapse. Reversible adsorption may occur as long as the interlayer space stays open. The irreversible adsorbed fraction was quantified and taken into account in the modeling approach to calculate the selectivity coefficient of each site.

Reactive transport of strontium in two laboratory-scale columns: Experiments and modelling

To support the environmental monitoring of nuclear sites, reactive transport models used to predict the migration of contaminants such as strontium-90 (⁹⁰Sr) in soils, sediments and aquifers are developed, continuously tested and improved. This study aims at assessing the adequacy of the multi-site ion exchanger model (MSIE) based on a component "additivity approach" and coupled to the advection-dispersion equation (ADE) to simulate Sr transport in a clayey sandstone and a Bt soil horizon. We have also compared the performance of the modelling approach with simulation results obtained by considering a K_d approach (constant K_d). Transport experiments were performed in centimetre- and decimetre-scale columns in order to test the model sensitivities to the mineral abundance and the specificities of their reactive parameters. Non-reactive transport experiments with conservative tracers allowed us to determine the transport parameters, such as porosity and dispersivity. In this paper, we have compared the Sr transport simulation results with Sr experimental breakthrough curves acquired at various flow velocities. The simulations results show that the K_d approach can reproduce experimental data in the case of the clayey sandstone when a certain amount of uncertainty is accepted, whereas the additivity approach better fits the Sr reactive transport in both columns (especially the maximum value) without it being necessary to adjust the parameters. These results advocate for more complex retention models than K_d to better understand and improve the robustness of Sr transport predictions. The clay content, the relative abundance of illite and smectite, and the clay mineral specificity, are all sensitive parameters when it comes to defining the reactive system involved in Sr transport simulation. The results highlight the influence of illite in the spreading of the Sr breakthrough curve, especially through its low-capacity and high-selectivity site. This implies having access to a robust and extensive set of retention parameters acquired on reference minerals. In this study, the results obtained for the clayey sandstone confirm the robustness of our selected parameters when clay minerals have similar reactivity levels as the reference minerals. This set of parameters appears more limited in the case of the Bt soil containing weathered or evolved minerals. The choice of modelling approach is therefore crucial for accurately modelling and predicting Sr transport behaviour in porous media, as is the representativeness of the minerals in the database.

Diffusion of organic anions in clay-rich media: Retardation and effect of anion exclusion

The transport of emerging organic contaminants through the geosphere is often an environmental issue. The sorption of organic compounds slows their transport in soils and porous rocks and retardation is often assessed by extrapolation of batch experiments. However, transport experiments are preferable to strengthen migration data and modelling. In this context, we evaluated the adsorption of various organic acids by means of through-diffusion experiments in a sedimentary clay-rich rock (Callovo-Oxfordian, East of Paris Basin, France). A low diffusivity of organic anions was quantified with effective diffusion coefficients, D_e, ranged between 0.5 and 7 10^-12 m² s^-1. These values indicated an organic anion exclusion. As for chloride, the porosity accessible to organic anions was lower than that of water: ε_a(organic anions) < ε(water). The partial exclusion of organic anions from rock porosity was linked to both charge and size effects. A significant retardation was observed for organic anions such as oxalate, citrate or α-isosaccharinate. Yet, retardation measured by diffusion experiments was significantly lower than expected from batch experiments on crushed samples. An empirical correction factor is proposed to account for a possible decrease of retardation with accessible porosity of diffusing solute. This feature has significant implications for the estimation of migration parameters of organic compounds in the environment.

Adsorption of polar organic molecules on sediments: Case-study on Callovian-Oxfordian claystone

The release and transport of anthropogenic organic matter through the geosphere is often an environmental criterion of safety. Sedimentary rocks are widely studied in this context as geological barriers for waste management. It is the case of Callovian-Oxfordian claystone (COx), for which several studies report adsorption of anthropogenic organic molecules. In this study, we evaluated and reviewed adsorption data of polar organic molecules on COx claystone. Experiments were performed on raw claystone, decarbonated and clay fractions. Adsorption isotherms were measured with adsorbates of various polarities: adipate, benzoate, ortho-phthalate, succinate, gluconate, oxalate, EDTA, citrate. A significant adsorption was observed for multidentate polycarboxylic acids as evidenced with phthalate, succinate, oxalate, gluconate, EDTA and citrate (R_d = 1.53, 3.52, 8.4, 8.8, 12.4, 54.7 L kg^-1 respectively). Multiple linear regression were performed as a statistical analysis to determine the predictors from these adsorption data. A linear correlation between adsorption data (R_d) and dipole moment (μ) of adsorbates was evidenced (R² = 0.91). Molecules with a high dipole moment, μ(D) > 2.5, displayed a significant adsorption, R_d≫1 L kg^-1. A qualitative correlation can be easily estimated using the water/octanol partition coefficient, P_ow, of adsorbates (R² = 0.77). In this case, two opposite trends were distinguished for polar and apolar molecules. The use of organic carbon content in sediments is relevant for predicting adsorption of apolar compounds, log (P_ow)>+1. The oxides/clays contents may be relevant regarding polar molecules, log (^apparentP_ow)<-1. The proposed scheme offers a general methodology for investigation of geo-barriers towards heterogeneous organic plumes.

Interlayer collapse affects on cesium adsorption onto illite

Cesium adsorption onto Illite has been widely studied, because this clay is especially relevant for Cs migration-retention in the environment. The objective of this study is to analyze how Cs adsorption onto Illite is affected by structural changes produced by the presence of different exchangeable cations--and specifically interlayer collapse. Cs sorption isotherms were carried out with Illite previously exchanged with Na, K, or Ca, at a broad enough range of ionic strength, for the determination of the possible affect of the electrolyte on the structure of Illite. In the presence of Ca, the maximum sorbed Cs was unexpectedly high (900 mequiv · kg(-1)) given the cationic exchange capacity commonly accepted for Illite (near 200 mequiv · kg(-1)). This was explained by the expansion of Illite layers (decollapse) induced by large hydrated cations such as Ca(2+) that may facilitate cation uptake--especially Cs(+), which is a highly selective cation. In the presence of Ca (and most probably of other divalent cations), Cs accessibility to exchange positions is increased. Both experimental evidence and the modeling of Cs sorption onto Illite supported the hypothesis of decollapse. Our results demonstrate the requirement of accounting for Illite decollapse especially for high Cs loadings, because of the potential prediction errors for its migration. Ignoring the Illite decollapse could lead the biased estimation of selectivity coefficients and consequently the erroneous prediction of sorption/migration behavior of Cs, and possibly other contaminants, in the environment.

Mobility of Na and Cs on montmorillonite surface under partially saturated conditions

Cs migration in soils at contaminated sites or in clay-rich backfill of waste disposal sites can take place under partially saturated conditions. To understand the molecular mechanism of Cs migration in partially saturated clays, Grand Canonical Monte Carlo simulations were applied to model adsorption of water films onto external surfaces of Cs and Na montmorillonites as function of partial water pressure. The surface complexation and diffusivity of Cs and Na at different partial water pressure was obtained by molecular dynamics simulations. The results suggest that ion mobility in adsorbed water films on external basal surfaces of clay is similar to that in the near-surface water of a saturated pore as far as the thickness of the adsorbed water film is more than two water layers. At lower partial water pressure (i.e., in thinner water films) the ion mobility dramatically decreases. In contrast, the average water mobility in thin water film is higher than in the water-saturated system due to enhanced mobility of water molecules close to vapor-film interface. The results of the simulations were applied to interpret recent laboratory measurements of tritiated water and Cs diffusivity in Callovo-Oxfordian Claystones under partially saturated conditions.